(1) In communications, interference (static) that destroys the integrity of signals on a line. Noise can come from a variety of sources, including radio waves, nearby electrical wires, lightning, and bad connections. One of the major advantages of fiber optic cables over metal cables is that they are much less susceptible to noise.
(2) In general, anything that prevents a clear signal or message from being transmitted. For example, you might hear someone complain of a lot of noise in a newsgroup, meaning that there are many superfluous messages that don't add anything to the discussion.
(3) In digital images and digital photography the term noise is used to describe the occurrence of color dots or specks where there should be none. For example, in a digital image of a pool of blue water, you may notice white, grey or other colored specks in the image where it should be blue. In scanning technology, image noise is usually referred to as grain or film grain.